What is the difference between def and defonce in Clojure?

Question

What is the difference between def and defonce in Clojure?

When to use def over defonce or vice versa?

Answer 1

defonce is skipped when variable is already defined.

user> (def a 1) ;;=> #'user/a
user> a ;;=> 1
user> (def a 2) ;;=> #'user/a
user> a ;;=> 2
user> (defonce b 1) ;;=> #'user/b
user> b ;;=> 1
user> (defonce b 2) ;;=> nil
user> b ;;=> 1

Answer 2

Defonce only binds the name to the root value if the name has no root value.

For example, like Jay Fields blogs about, it can be used in conjunction when you want to reload namespaces but you might not need to reload all.

(defonce ignored-namespaces (atom #{}))

(defn reload-all []   
  (doseq [n (remove (comp @ignored-namespaces ns-name) (all-ns))]
    (require (ns-name n) :reload )))